Building and construction materials based on hydraulic and non-hydraulic binders are examples where composite fibers are employed to modulate the physical properties according to specific needs. Concrete and mortar are relatively brittle materials where the tensile strength is typically much lower compared to the compressive strength of the material. Therefore, under normal circumstances concrete needs to be reinforced usually with steel reinforcing bars. It has become increasingly popular to additionally reinforce concrete or mortar with short randomly distributed fibers of various types to satisfy the needs of modern building industry. The main purpose is not only to increase the toughness (resistance to cracking) of the resulting inorganic binder compositions, but also to improve the tensile strength (crack strength) and ductility of the building materials.
Mortar is a mixture of fine aggregates with hydraulic cement, whereas concrete additionally contains coarse aggregates. The cement constituent is used as a synthetic inorganic material making up the matrix into which the aggregates are embedded. Concrete and mortar mixtures may also contain pozzolans and other admixtures commonly utilized for conventional and specialty uses thereby modifying the physical properties of the unhardened and hardened inorganic binder compositions. Cement typically includes anhydrous crystalline calcium silicate (C3S and C2S), lime and alumina. In the presence of water the silicates react to form hydrates and calcium hydroxide. The hardened structure of cement depends on a three dimensional nature and complex arrangement of newly formed crystals that intrinsically depends on the quantities of the ingredients, curing time and composition of the concrete aggregates. In the course of the hardening process plastic, chemical or dewatering shrinkage may create voids causing defects and shrinkage cracks. Moreover sulfate attack in concrete and mortar often is the cause of internal pressure producing cracks in the material and in consequence destabilizes structures made of such material. Sulfate attack can be either ‘external’ or ‘internal’, i.e. due to penetration of external sulfates in solution into the concrete or due to a soluble source being incorporated into the concrete at the time of mixing for example. The more common type of sulfate attack is external and typically occurs by penetration of water containing dissolved sulfate. The changes caused by external sulfate attack may vary in type or severity but commonly include extensive cracking and loss of bond between the cement paste and aggregate most likely due to crystallization of ettringite. The effect of these changes is an overall loss of concrete strength. Internal sulfate attack on the other hand occurs where a source of sulfate is found in one of the concrete ingredients. This may occur through the use of sulfate-rich aggregate, excess of gypsum added to the cement or by contamination. Under special circumstances such as elevated temperatures during hardening of concrete ettringite crystallization causes expansion and cracking of the matrix and subsequently serious damage to the concrete structures.
In the process of counteracting potential defects fibers have been introduced to the inorganic binder compositions to reinforce the final matrices. Interfacial bond strength governs many important composite properties, such as overall composite strength, ductility, energy absorption property etc. Many endeavors have been undertaken to enhance or increase the bonding capacity and compatibility at the interface of fibers to matrices in various composite materials and concrete in particular. A variety of fibers, natural and synthetic, have been used in inorganic binder compositions to increase the stability of resulting structural elements made for example from concrete mixtures. A non-limiting list of examples for such fibers are from natural materials, such as cellulose-based fibers, like cotton, viscose, hemp, jute, sisal, abaca, bamboo, cellulose, regenerated cellulose (e.g. Lyocell®), from synthetic materials like polyamide, polyester, polyacrylonitrile, polypropylene, polyethylene, polyvinylalcohol, aramide, polyolefines in general, but also from inorganic mineral or metal-based materials like carbon, glass, mineral wool, basalt, oxide ceramic and steel.
Fibers of various shapes and sizes produced from such materials are being used as stabilizers and reinforcing elements, however, for most applications such as structural and nonstructural purposes, steel fibers are most commonly used. Fibers are usually randomly oriented in the matrix. Examples of commonly used synthetic fibers are polypropylene, polyethylene and polyvinyl alcohol, all of which suffer from one or more problems, such as high cost (e.g. polyvinylalcohol), low tenacity or low interfacial bonding (e.g. polypropylene).
When concrete or mortar mixtures contain fibers there is a considerable improvement of post-cracking behavior. Compared to plain concrete, fiber reinforced concrete is much tougher and impact resistant. Plain concrete fails suddenly once the deflection corresponding to the ultimate flexural strength is exceeded. Fiber reinforced concrete continues to sustain considerable loads even in excess of fracture deflection of plain concrete. This is due to the fact that fibers significantly alter the energy absorption properties of the inorganic binder compositions. (Swamy R N et al., Materiaux et Constrctions Vol. 8, 45, 235-254, 1975; Kim, Y Y et al., ACI Structural Journal, Vol. 101, 6, 792-801, 2004; EP 0,225,036; EP 2,557,185). The most outstanding property of the inorganic binder compositions is the potential for crack arrest and crack control mechanisms. This further directly affects the improvement of other properties linked to cracking such as strength, stiffness, ductility, fatigue, thermal loading, resistance to impact and energy absorption. Crack-control therefore seems to be the most important aspect when considering reinforcement of cementitious based inorganic binder compositions.
A limitation in the use of most fibers as reinforcement agents is a result of the low pull-out strength based on poor wettability and adhesion to the matrix (low interfacial bonding) and to cementitious material in particular. Failure of fiber-reinforced concrete is primarily due to fiber pull-out or de-bonding. Therefore failure of fiber reinforced concrete will not occur suddenly after initiation of a crack. Since the bonding of fibers to the matrix is mainly mechanical, literature indicates that to obtain good adhesion between fiber and matrix material it is usually necessary to carry out pretreatments, chemically as well as physically. A variety of mechanisms are known and described in the literature and are employed to increase the interfacial bonding of fibers to inorganic binder compositions (Li V. C. et al., Advanced Cement Based Materials, 1997, Vol. 6, 1-20). Increasing the fiber surface area is for example one way to increase the area of interaction between fiber and matrix. This increase in surface area enhances the mechanical bond to the matrix and can for example be achieved by fibrillation procedures. Further surface modulations of fibers have been utilized that lead to improvement of matrix-fiber interaction and mechanical bonding such as twisting, embossing crimping and introduction of hooks into fibers to mention a few measures generally employed.
Other means of surface modification also lead to enhancement of adhesion between fiber and matrix. Plasma treatment is utilized to introduce polar groups onto the surface thereby increasing the reactivity and wettability of the fiber (U.S. Pat. No. 5,705,233). This leads to an improved compatibility and bonding to cementitious matrix ultimately resulting in increased pull-out strength of the respective fibers.
Special techniques have been developed to increase the mechanical bond to the matrix and assure advantageous composite properties. The geometry of the fiber influences the bond between the fiber and matrix structure, e.g. fibers of three dimensional shape demonstrate improved bonding properties (Naaman A. E., Mcgarry F. J., Sultan, J. N.—Developments in fiber-reinforcements for concrete, Technical Report, R 72-28, School of Engineering, MIT, May 1972, p. 67).
Synthetic fibers offer a number of advantages as reinforcement agents in concrete. They present high elastic modulus and are cheap. EP 0,225,036 discloses a method of making polypropylene fibers antistatic and thus increasing the hydrophilicity whereby the embedding of the fibers in the matrix and uniform distribution is improved. Further disclosed are methods for improving the embedding properties of polypropylene fibers by crimping, roughening or profiled shaping of the fibers.
WO 97/39054 discloses individual fiber bodies having ettringite formed on at least a portion of their surface. Ettringite crystals are precipitated in situ within an aqueous medium onto the surface of hard wood fibers in order to improve the compatibility of the utilized wood fiber within a hydraulic matrix. Further disclosed is the use of the wood fibers to reinforce inorganic binder compositions and to enhance bond strength between the fibers and cementitious matrices.
DE 3602310 discloses the pretreatment of individual fiber bodies with silicic acid aerosol particles (silica fume). The amorphous silica fume particles are deposited on the fiber surface from an aqueous dispersion in the presence of dispersants prior to using the fiber in cementitious binder systems. In partucluar DE 3602310 discloses the use of silica fume particles to prevent direct interaction of cement hydrate products with the fiber and thereby prevent or reduce ageing and/or deterioration of the fiber in the resulting composite material. Despite the measures employed to increase the bonding of fibers to the matrix the utilization of individual fiber types are still limited because for high-tech and demanding applications the respective pull-out strength is still low and insufficient to satisfy the needs of high performance concrete materials. Further, individual techniques available are restricted to only limited fiber materials, i.e. solely to mineral-based, polymer-based or even only to a selected, individual material species thereby limiting the general and widespread use of individual techniques.
The hydrophobicity of a variety of fibers for example and respective low wettability and hence low adhesion to cement matrix is one of the major problems that prevent widespread and large scale use of cheap polymeric material such as polypropylene.
It would therefore be favorable to have a method at hand to easily modify and further improve the bonding characteristics of such fibers to a construction or building materials, in particular that of polypropylene fibers in non-hydraulic and hydraulic inorganic binder compositions. The problem to be solved by the present invention is to provide means to increase the pull-out strength of fibers used in building and construction materials based on non-hydraulic, latent hydraulic and hydraulic binders and so enhance strength and flexibility with sustained mechanical stability of said materials.